The use of motors in bore hole drilling, especially in drilling for oil and gas but also in mining operations, has been a standard procedure in the art. Such motors are employed to rotate drills for boring in the earth both for forming a bore hole and also for coring. The motors are also useful in oil field operations, such as tube cleaning, milling operation, cement drilling and other operations where it is desired to rotate a rod at the end of which a tool is to be rotated. We refer to such motors as in-hole motors when designed to be run at the end of a pipe and adjacent to the drill bit. In the usual case, the rotor of the motor and the drill bit rotate with respect to a stator which, in turn, is connected to a conventional drill string composed, in the case of the drilling of well bores, of a "kelly," drill pipe, and drill collar as required. This string extends to the surface with the kelly mounted in the rotary table. Where the in-hole motor is a hydraulic motor used as an in-hole motor in drilling, the liquid is the usual drilling fluid, i.e., mud or gas. It serves its usual function in the drilling operation, returning to the surface carrying the detritus, i.e., cuttings resulting from the drilling operation. However, in this combination, the circulating mud has an additional function, and that is to supply the hydraulic power to operate the hydraulic motor.
One of the primary problems resides in the design of the bearing system which will permit operations for periods of economic length.
It has been conventional to rely on a part of the circulating mud to pass through the bearings to lubricate them. Such bearing systems are shown in E. P. Garrison et al, U.S. Pat. No. 3,516,718, issued Jan. 23, 1970, and in Garrison et al U.S. Pat. Nos. 3,456,746 and 3,857,655 issued Dec. 31, 1974. Mud lubrication of bearings has also been applied to turbine-operated drills of the prior art.
When mud-lubricated bearings are employed with motors of the helicoidal type, such as have been employed in the prior art in-hole motors, problems arise with respect to limiting the flow of mud through the bearings; and problems arise from the eccentric motion of the rotor. Such motors are shown in Clark U.S. Pat. No. 3,112,801, patented Dec. 3, 1973 and the above U.S. Pat. No. 3,857,655.
The prior art solutions for limiting the bypass of mud through the bearings are shown in the Garrison patents. These include the provision of a grooved rubber radial bearing which also acts as a flow restrictor to limit the fluid bypassing through the bearings so as not to rob unduly the main flow through the bit nozzles required to provide the necessary flow to remove the cuttings.
Since the rotor of the motor rotates in an eccentric manner, it is necessary to convert this motion into a true rotation about a fixed axis so that the bit may be rotated in the proper manner. This is accomplished by connecting the end of the rotor to a connecting rod by means of a universal joint and connecting the connecting rod to a drive shaft by means of a second universal joint.
Further, while the universal joints do a fairly good job in the case of the helicoidal motors of converting the eccentric motion of the rotor to a rotary motion, there remains a residual force on the drive shaft which is transverse to the axis of rotation. This transverse force is periodic in direction, reversing itself on each reversal of the eccentric motion. Additionally, when drilling in steeply dipping formations or in drilling out dog legs, or in drilling deviated holes, particularly when using bent subs, or bent connecting rod housings at the connecting rod, a thrust is encountered at the bit which is transverse to the bit axis. The result is a transverse displacement of the shaft and a transverse force applied to the radial bearing employed, for example, the rubber bearing referred to or any other radial bearing which may be employed.
Problems have arisen in such prior art combination. The rubber radial bearings, which even in the first place, due to molding limitations, do not act adequately to restrict the amount of bypass, deteriorate in use and result in premature failure. This failure includes erosion of the bearing passageways where the grooves are washed out.
Circulating mud usually contains fine particles of "sand" resulting from the drilling operation. The mud returning up the annulus is separated from the cuttings, but some fine particles produced by the drilling operation escape in the treated mud. The returning mud passing at high volumetric velocity through the grooves in the rubber flow restrictor erodes the grooves. The result is that the pressure drop through the restrictor is reduced and a large portion of the input mud is bypassed.
The percentage of the fluid bypassed, even with newly formed radial bearings, may be excessive because it is difficult to mold such bearings to form passageways through the bearings that will have the desired flow resistance and yet provide a suitable bearing surface which will not have excessive frictional resistance. The erosion of the rubber by the mud is also a problem.
Experience has shown that an eroded marine bearing employed as a radial bearing permits an excessive flow through the bearing flutes, under the above flow conditions. Such flow rates may range up to about 20% of the total volumetric flow rates. This is an excessive bypass flow. In order to reduce the flow, a separate flow restrictor is added, as is shown in the above Garrison U.S. Pat. No. 3,456,746. This may reduce the flow in the range of about 5 to 10% of the total flow, depending on the magnitude of the volumetric flow rate of the mud. The greater the percentage of the bypass, the greater the volumetric flow rate.
It is to be recognized that the pressure drop between the stator discharge to the annulus exterior of the drill may be in the order of 200 to 1500 pounds per square inch and the volumetric rate of flow from 50 to about 600 gallons per minute, depending upon the depth, nature of the mud, size of the tool, and designs of the nozzles of the bit.
Excessive bypass flow through the bearing system imposes excessive erosion of the thrust bearings. A bypass flow has been experienced, in the prior art, of about 5 to about 30 gallons per minute, that is, about 5 to about 10% of the volumetric flow rate in the range of the pressure drops referred to above. An increase in volume flow through the marine bearing, flow restrictor, and thrust-bearing packages may thus rise to excessive magnitude.
The pressure drop and volume rate of flow of the mud through the motor depend on the horsepower requirement and rpm of the drilling effort. This establishes the gallons per minute of mud that must be circulated. The mud input pressure is fixed by the total pressure drop through the drill string, the hydraulic motor, bit nozzles and annulus pressure drop. The volume bypassed through the bearings is subtracted from the flow through the nozzles. The pump must provide for sufficient input to supply the required flow rate and pressure drop. The bit manufacturer usually supplies the nozzle pressure drop to give the required lifting effect and cutting action. Furthermore, the depth to which a well may be serviced by a given pump assembly and therefore the limit of bit advance depend on the permissible horsepower required to move the mud through the motor to and through the bit nozzles and return the cuttings to the surface. Any additional demand on the pump, required to supply excessive bypass, is a limitation on the depth to which a given drilling rig can go. Additional pump capacity is thus required.
It is difficult to build a rubber bearing which is so finely tuned as to meet these parameters and not permit an excessive flow through the bearings. Furthermore, as has been stated above, pressure drops tend to erode the passageways in the rubber bearing so that they do not for long retain their original cross-sectional areas.